Existence of Barrow-wights is connected with necromancy; from "Morgoth's Ring" of the History of Middle-earth series:
But it would seem that in these after-days more and more of the Elves, be they of the Eldalië in origin or be they of other kinds, who linger in Middle-earth now refuse the summons of Mandos, and wander houseless in the world, unwilling to leave it and unable to inhabit it, haunting trees or springs or hidden places that once they knew. Not all of these are kindly or unstained by the Shadow. Indeed the refusal of the summons (of Námo to Mandos) is in itself a sign of taint.
It is therefore a foolish and perilous thing, besides being a wrong deed forbidden justly by the appointed Rulers of Arda, if the Living seek to commune with the Unbodied, though the houseless may desire it, especially the most unworthy among them. For the Unbodied, wandering in the world, are those who at the least have refused the door of life and remain in regret and self-pity. Some are filled with bitterness, grievance, and envy. Some were enslaved by the Dark Lord and do his work still, though he himself is gone. They will not speak truth or wisdom. To call on them is folly. To attempt to master them and to make them servants of one’s own will is wickedness. Such practices are of Morgoth; and the necromancers are of the host of Sauron his servant.
Some say that the Houseless desire bodies, though they are not willing to seek them lawfully by submission to the judgement of Mandos. The wicked among them will take bodies, if they can, unlawfully. The peril of communing with them is, therefore, not only the peril of being deluded by fantasies or lies: there is peril also of destruction. For one of the hungry Houseless, if it is admitted to the friendship of the Living, may seek to eject the fëa from its body; and in the contest for mastery the body may be gravely injured, even if it be not wrested from its rightful habitant. Or the Houseless may plead for shelter, and if it is admitted, then it will seek to enslave its host and use both his will and his body for its own purposes. It is said that Sauron did these things, and taught his followers how to achieve them.
Such elvish wights could be quite large in numbers (Avari elves were numerous, many could fall under Dark Power of Morgoth or Sauron). And of course there were always those who practiced sorcery (Black Númenóreans, other peoples), many of them. "Host of Sauron" implies large numbers.